The present invention relates to a liquid level detection device, and more particularly a liquid level detection device which is suitable for detecting a residual fuel amount of a vehicle such as a motor vehicle which uses an electrolytic fuel such as alcohol itself like ethanol or methanol or alcohol-based fuel which contains such alcohols.
An example of a conventional liquid level detection device is shown in FIG. 9. A liquid level detection device 101 shown in FIG. 9 has a sensor 102 disposed within a container which stores a liquid therein. The sensor 102 has a variable resistance 103 which changes a resistance value by allowing a contact to move in association with a change in liquid level to thereby output a voltage signal according to a resistance value of the variable resistance 3 when a predetermined voltage is applied to the sensor. In addition, the liquid level detection device 101 includes a power supply circuit 104 which applies a voltage to the sensor 102 (for example, refer to Patent Document No. 1).
An example of the configuration of the sensor 102 is shown in FIG. 10. The sensor 102 shown in FIG. 10 includes an arm 111 to a distal end of which a float 110 adapted to float in the liquid is attached, a frame 112 which supports rotatably a proximal end portion of the arm 111, a circuit board 113 mounted on the frame 112 and a contact element 114 provided at the proximal end portion of the arm 111. A resistor, not shown, is formed on a surface of the circuit board 113, and a predetermined voltage is applied to this resistor by the power supply circuit 104. The contact element 114 swings about an rotational axis of the arm 111 in association with rotation of the arm 111 and slides over the resistor on the circuit board 113 (for example, refer to Patent Document No. 4).
One end of the resistor of the circuit board 113 and the contact element 114 are electrically connected to output terminals 115, 116 of the sensor 102, respectively. The float 110 fluctuates while linking with a change in liquid level, and the arm 111 rotates in association with fluctuation of the float 110, causing the contact element 114 to slide over the resistor, whereby a resistance value of a portion of the resistor which lies between the end where the resistor connects to the output terminal 115 to the contact element 114 (hereinafter, referred to as a first region of the resistor) changes. Namely, the variable resistance 103 is made up of the resistor and the contact element 114. The voltage applied to the resistor is divided according to the resistance value of the first region of the resistor, and the voltage applied to this first region is detected between the output terminals 115, 116 as an output signal of the sensor 102.
Referring to FIG. 9 again, the liquid level detection device 101 obtains the output signal of the sensor 102 at a processing circuit 105 and drives a gauge 106 which indicates a residual amount of the liquid based on the signal so obtained.
Incidentally, when detecting a residual amount of fuel of a vehicle such as a motor vehicle, the sensor 102 of the liquid level detection device 101 is disposed within a fuel tank, and in recent years, bio-ethanol or the like is used for fuel of a vehicle. Alcohol-based fuel such as bio-ethanol is an electrolytic liquid, and when the sliding contact surfaces of the resistor and the contact element 114 of the sensor 102 are exposed to the electrolytic liquid, an electrochemical reaction such as electrolytic corrosion is produced on the sliding contact surfaces as the voltage is applied thereto, as a result of which the sliding contact surfaces are deteriorated, and this raises the contact resistance at the contact portion, interrupting an accurate measurement. In order to avoid the drawback like this, there is proposed a technique in which a voltage is applied to the sensor intermittently in a predetermined cycle (for example, refer to Patent Document Nos. 1 to 3).
[Patent Document No. 1]JP-A-2002-214023[Patent Document No. 2]JP-A-63-138215[Patent Document No. 3]JP-A-2006-214828[Patent Document No. 4]JP-B-3898913
Here, although the conducting time (energizing time) constitutes an important factor in electrochemical reaction such as electrolytic corrosion, in Patent Document Nos. 1 to 3 above, nothing is stated about a preferable conducting time per cycle, and no sufficient countermeasures has been taken in those related art liquid level detection devices.